Add first OSX Travis test.

.travis.yml

@@ -37,6 +37,9 @@ matrix:
     - compiler: gcc
       env: TOOLSET=gcc-5.1
       os: linux
+    - compiler: clang
+      env: TOOLSET=xcode-6.1
+      os: osx
 
 before_install: wget "https://raw.githubusercontent.com/boostorg/regression/develop/ci/src/script.py"
 install: python script.py install